Sealed ink cup pad printing machines comprise an ink cup which is supported in inverted fashion with a sealing and doctoring end surface thereof in abutment with a printing block or "cliche" that is mounted in reciprocating fashion for transferring ink in a predetermined pattern to a printing pad. The ink cup includes an annular surface, which may be an integral part of the cup or, alternatively, a separate ring, that serves as a sliding seal between the ink cup and the cliche and as a doctor blade or "knife" for ensuring that only the engraved portions of the cliche carry ink to the printing pad pick-up site.
U.S. Pat. Nos. 4,557,195 and 4,905,594 disclose examples of prior such machines and their disclosures are incorporated herein by this reference.
To ensure quality printing with pad printers, it is important that the annular doctor blade of the ink cup reliably scrape or wipe from the cliche plate all ink that is not within the engraving recesses. Consistently obtaining the clean wiping action has presented problems. To obtain and maintain a sealing and wiping action, doctor elements typically have been formed of a very hard material, such as carbide, ceramic, high speed steel, or other hard metal, and have been finished to a very accurate planar surface, as by lapping. For example, the aforenoted U.S. Pat. No. 4,557,195 describes the use of hard materials for forming the end contact surfaces of the ink cups, at least in those areas which serve a wiping function. A further suggestion is made therein that it may be possible to use elastic parts made of metal or plastic for the side portions of rectangular cup end surfaces which extend parallel to the direction of displacement and do not have a wiping function, but merely serve the function of sealing aprons in a non-etched area.
Many cliches are made of metal, particularly for high volume reproduction of the same image. However, cliches which have a plastic gravure surface, e.g. of a photosensitive polymer material, have gained wide usage because generally they are much less expensive to produce and to engrave than the cliches which use a metal gravure surface. The plastic gravure surfaces may be provided by using a basic support plate or block, as of metal, with a gravure surface formed by a layer, laminate or coating of a photosensitive polymer, or may constitute an entire plate or block of such a polymer material. However, the plastic gravure surfaces have tended to wear much more rapidly than the metal or metal-surfaced cliches. For this reason, the cliches with plastic gravure surfaces have been used primarily for relatively short production runs.
Ink cups have generally been formed of metals such as aluminum, steel, or plastics. The doctoring portion of the cup is ordinarily constructed of carbide steel. U.S. Pat. No. 5,662,041 discloses an ink cup wherein the doctoring portion is formed of a polymeric material, such as a polymeric composite containing polyaryletherketone and carbon fibers, permeated by a solid lubricant.
The ink cup, the doctoring portion, and by extension, the material(s) of which it is constructed, must exhibit certain physical properties. They must be chemically resistant to components of the ink, notably dibasic esters and aromatic hydrocarbons. They must also be sufficiently strong and stiff in view of the demanding operating environment. Furthermore, the materials used to construct the doctoring portion must be resistant to wear, as the doctoring portion repeatedly wipes the cliche during operation.